Effect of wall–molecule interactions on electrokinetic transport of charged molecules in nanofluidic channels during FET flow control

Abstract

The interactions between charged molecules and channel surfaces are expected to significantly influence the electrokinetic transport of molecules and their separations in nanochannels. This study reports the effect of wall-molecule interactions on flow control of negatively charged Alexa 488 and positively charged Rhodamine B dye molecules in an array of nanochannels (100 nm wx 500 nm dx 14 mm l) embedded in fluidic field effect transistors (FETs). For FET flow control, a third electrical potential, known as a gate bias, is applied to the channel walls to manipulate their zeta-potential. Electroosmotic flow of charged dye molecules is accelerated or reversed according to the polarity and magnitude of the gate bias. During FET flow control, we monitor how the electrostatic interaction between charged dye molecules and channel walls affects the apparent velocity of molecules, using laser-scanning confocal fluorescence microscopy. We observe that the changes in flow speed and direction of negatively charged Alexa 488 is much more pronounced than that of positively charged Rhodamine B in response to the gate bias that causes either repulsive or attractive electrostatic interactions. This observation is supported by calculations of concentration-weighted velocity profiles of the two dye molecules during FET flow control. The velocity profile of negatively charged Alexa 488 is much more pronounced at the center of each nanochannel than near its walls since Alexa 488 molecules are repelled from negatively charged channel walls. This pronounced center velocity further responds to the gate bias, increasing the average velocity by as much as 23% when -30 V is applied to the gate (zeta-potential = -80.6 mV). In contrast, the velocity profile of positively charged Rhodamine B is dispersed over the entire channel width due to dye-wall attraction and adsorption. Our experimental observations and calculations support the hypothesis that valence-charge-dependent electrostatic interaction and its manipulation by the gate bias would enhance molecular separations of differentially charged molecules in nanofluidic FETs.

Impact of leakage current and electrolysis on FET flow control and pH changes in nanofluidic channels

“…This device allows the use of laser-scanning confocal fluorescence microscopy (LS-CFM) and multiple internal reflection Fourier transform infrared spectroscopy (MIR-FTIRS). In our previous publication,23 these techniques were used to probe wall–molecule interactions and their impact on ζ -potential. In this study, we have used these techniques to monitor the FET flow control of fluorescent dye molecules as well as the shift in solution pH in response to the transverse gate potential.…”

Shift of isoelectric point in extended nanospace investigated by streaming current measurement

“…Many unique ion transport phenomena have been reported due to surface charge effects, such as ion enrichment, ion rectification, and ion concentration. [1][2][3][4] At present, much research is focused on ion transport behavior based on the surface charge effect.…”

Fabrication of Nanofluidic Biochips with Nanochannels for Applications in DNA Analysis

“…The electrical field distribution and current leakage in these nanofluidic devices were also investigated in details 36, 37. Furthermore, a fluidic field effect transistor based on these nanochannel structures was fabricated to probe the transport of charged molecules and to measure the pH shift in nanochannels in response to an electrical potential applied to the gate 38, 39…”

“… Cross‐sectional SEM images of nanochannels: (A) nanochannels after plasma etching; (B) SiO 2 ‐covered nanochannels after thermal oxidation; and (C) nanochannels sealed with a Pyrex cover after anodic bonding 38. Copyright 2009, Royal Society of Chemistry.…”